1. Field of the Invention
This invention relates to a hydraulic actuator and control system for generating and transmitting acoustic vibration signals through a drillstem from a downhole location to a surface receiver system.
2. Background
The desire to obtain data pertaining to downhole operating conditions when drilling and completing oil and gas wells has spawned the development of several types of systems for transmitting information through the wellbore or the drillstring to a surface receiving and recording system. One promising approach to transmitting signals from downhole measurement devices to the surface comprises generating mechanical vibrations or so-called acoustic waves which are transmitted through the drillstem to a surface receiving system connected to the drillstem. Acoustic wave generators or transmitters are particularly attractive in view of the development of surface disposed receiving systems of the type disclosed in U.S. Pat. No. 4,715,451 to Bseisu, et al., and assigned to the assignee of the present invention. The system disclosed in the Bseisu, et al., patent is adapted to receive and transmit to a suitable display or recorder acoustic waves generated in axial, torsional and bending modes of the drillstem. Accordingly, if a suitable transducer or generator can be disposed downhole, then certain measurement parameters such as pressure and temperature conditions can be converted to vibrational signals which are transmitted through the drillstem for receipt by a system such as disclosed in the patent reference.
One system, generally of the type described above, is disclosed in U.S. patent application Ser. No. 07/554/022, filed Jul. 16, 1990 in the name of Melvin G. Montgomery and also assigned to the assignee of the present invention. Another type of transmitter system is described in U.S. Pat. No. 4,992,997 to Bseisu and assigned to the assignee of the present invention.
One shortcoming of some prior art systems is that the energy input to the drillpipe or tubing by the transducer or generator, particularly in long drillstems or tubing strings, may not be sufficient to obtain signals at the surface which are coherent due to transmission losses in the pipe or tubing. The present invention seeks to overcome this problem by providing a variable frequency, hydraulically actuated acoustic wave generator and associated control system for transmitting acoustic signals through drillstems, tubing strings and the like.